<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<script>
  function fn(n){
      if (n===-1){
          return 1;
      }
      if (n===0){
          return 0;
      }
      return (fn(n-2)+fn(n-1))
  }
  var a = fn(10)
  document.write(a+'<br>')

  var b = fn(15)
  document.write(b+'<br>')
</script>
</body>
</html>